1. Field of the Invention
The present invention relates to a surface potential sensor for being positioned to face a charged member for detecting a surface potential of the charged member.
2. Description of the Related Art
In apparatus utilizing static electricity, such as PPC copying machines, page printers and electrostatic dust collectors, surface potential sensors have been heretofore employed to detect a surface potential of a charged member and keep the surface potential stable.
The principle of measuring a surface potential of a charged member will be described with reference to FIG. 5. In FIG. 5, when a detecting electrode is placed near a charged member which is to be measured and has a surface potential Vx, an electrostatic capacitance C is produced between the detecting electrode and the charged member to be measured. By periodically changing the distance between the detecting electrode and the charged member to be measured, the electrostatic capacitance C is changed correspondingly and a resulting voltage across a resistor R is output as Vout.
FIG. 6 is a block diagram of a surface potential sensor for measuring a surface potential of a charged member to be measured in accordance with a signal output from the circuit shown in FIG. 5. The distance between the detecting electrode and the charged member to be measured is periodically changed by using a tuning fork, for example, as shown in FIG. 6. A piezoelectric tuning-fork oscillator circuit vibrates the tuning fork with a constant amplitude. An initial-stage input circuit performs impedance conversion by receiving a voltage output from the detecting electrode under high input impedance and outputting a detection signal under low output impedance. A succeeding-stage amplifier circuit amplifies an AC component of the detection signal, a rectifier circuit rectifies the AC-amplified signal, and a DC amplifier circuit amplifies the rectified signal with a constant gain before outputting it as a measured voltage. A stabilized power supply circuit supplies stabilized source voltages to the various circuits.
FIG. 7 is a circuit diagram including the initial-stage input circuit and the succeeding-stage amplifier circuit in FIG. 6. As shown in FIG. 7, the initial-stage input circuit is of the source follower type made up by connecting a drain of an FET to a source voltage Vc, a resistor R3 between a source of the FET and ground, and a resistor R2 between the detecting electrode and a gate of the FET. Additionally, a resistor R1 is connected between the detecting electrode and ground. An operational amplifier OP1 has a non-inverted input terminal to which are applied a reference source voltage Vr through a resistor R5 and the AC component of a signal from the FET source through a capacitor C1. A voltage dividing circuit comprised of resistors R6, R7 is connected to an output terminal of the operational amplifier OP1. Further, a feedback circuit comprised of the voltage dividing circuit, resistors R8, R9 and capacitors C2, C3 is associated with the operational amplifier OP1. The succeeding-stage amplifier circuit using an operational amplifier is thus constructed.
The conventional surface potential sensor including the initial-stage input circuit and the succeeding-stage amplifier circuit, shown in FIG. 7, has a problem which can occur if noise is superposed on the reference source voltage Vr. Specifically, the operational amplifier OP1 amplifies a difference between the FET output (AC component) and the reference source voltage Vr. Accordingly, if noise is superposed on the reference source voltage Vr for some reason, the operational amplifier OP1 amplifies the noise component as well, resulting in a risk that detection accuracy of the surface potential may be lowered, or that a detection characteristic may be deteriorated. In particular, because the operational amplifier OP1 of the succeeding- stage amplifier circuit shown in FIG. 7 must amplify a weak voltage signal on the order of 10 .mu.V-100 mV, the effect caused by the noise superposed on the reference source voltage Vr becomes relatively large.